1. Field of the Invention
The present invention relates to a vibration damping system interposed between a vibratory body and a supporting body, and more particularly to an improvement of the vibration damping system including a device having two fluid chambers which are communicated through a hole with each other.
2. Description of the Prior Art
Recently a variety of vibration damping systems have been proposed to be disposed between a vibratory body such as a power unit and a supporting body such as a vehicle body in order to effectively damp the vibrations from the vibratory body. One of them is a power unit mounting device, such as the one disclosed in Japanese Patent Provisional (First) Publication No. 58-61345, through which the power unit is mounted on the vehicle body. The mounting device is provided with expandable two fluid chambers which are communicated through an orifice with each other for the purpose of improving vibration damping effect.
In such a power unit mounting device whose vibration damping effect is considered to be made due to resistance of fluid passing through the orifice, a so-called loss factor (tan .delta.) as a parameter of damping force is regarded as important like in a power unit mounting device made of a more rubber block. In this regard, the tuning of the mounting device is so made by alternating the diameter and the length of the orifice, that the maximum loss factor frequency is brought into coincidence with the frequency (about 10 Hz) of engine shake.
However, experiments have revealed that the power unit mounting device of the fluid filled type exhibits in practice vibration damping effect under the dynamic damper action of fluid within the orifice. Accordingly, in order to bring out the maximum dynamic damper effect, it is necessary to largely shift the resonance frequency of the fluid 5-10 Hz from the loss factor maximum frequency in which the resonance frequency is shifted 50-100 percent relative to the engine shake frequency. As a result, the fluid filled power unit mounting device cannot sufficiently exhibit its vibration damping ability and therefore cannot sufficiently reduce engine shake, thereby deteriorating an improvement effect in comfortableness to ride in a vehicle. This is shown in FIG. 10 of the present application, where a dotted curve f represents the vibration damping effect of a conventional power unit mounting device, such as that disclosed in Japanese Patent Provisional (First) Publication No. 58-61345, in which the maximum loss factor frequency is brought into coincidence with the engine shake frequency. It is notable that the vibration damping effect of the conventional power-unit mounting device deteriorates particularly in the frequency range where engine shake occurs.